Biological testing for the presence and/or concentration of an analyte may be conducted for a variety of reasons including, amongst other applications, preliminary diagnosis, screening samples for presence of controlled substances and management of long term health conditions.
Lateral flow devices (also known as “lateral flow immunoassays”) are one variety of biological testing. Lateral flow devices may be used to test a liquid sample such as saliva, blood or urine, for the presence of an analyte. Examples of lateral flow devices include home pregnancy tests, home ovulation tests, tests for other hormones, tests for specific pathogens and tests for specific drugs. For example, EP 0 291 194 A1 describes a lateral flow device for performing a pregnancy test.
In a typical lateral flow testing strip, a liquid sample is introduced at one end of a porous strip which is then drawn along the strip by capillary action (or “wicking”). A portion of the lateral flow strip is pre-treated with labelling particles that have been activated with a reagent which binds to the analyte to form a complex (if the analyte is present in the sample). The bound complexes and any unreacted labelling particles continue to propagate along the strip before reaching a testing region which is pre-treated with an immobilised binding reagent that binds bound complexes of analyte and labelling particles and does not bind unreacted labelling particles. The labelling particles have a distinctive colour, or other detectable optical property such as fluorescence. The development of a concentration of labelling particles in the test regions provides an observable indication that the analyte has been detected. Lateral flow test strips may be based on, for example, colorimetric labelling using gold or latex nanoparticles. Fluorescent colorimetry employs marker molecules which fluoresce a specific colour.
Another variety of biological testing involves assays conducted in liquids held in a container such as a vial, a PCR well or plate, a cuvette or a microfluidic cell. Liquid assays may be measured based on colorimetric measurements in reflection, transmission or fluorescence arrangements. An advantage of some liquid based assays is that they may allow tests to be conducted using very small (e.g. picolitre) volumes. However, in such small volumes, the desired colour change or fluorescence may be difficult to detect.
Sometimes, merely determining the presence or absence of an analyte is desired, i.e. a qualitative colorimetric test. In other applications, an accurate concentration of the analyte may be desired, i.e. a quantitative colorimetric test. Mobile devices including cameras, for example smart phones, have been widely adopted. It has been suggested to employ such mobile devices to perform quantitative analysis of the results of colorimetric lateral flow tests.